Previous designs for discrete voltage input circuits were only capable of accepting inputs over a specific narrow range of voltage levels, and were inaccurate and unreliable over a desired operating temperature range. A different circuit configuration was required for each specific narrow range of voltage levels, and/or jumpers, switches, firmware, etc., was required to reconfigure the input circuit to meet the application voltage requirement.
Referring to FIG. 1, depicted is a schematic diagram of a prior art voltage input circuit for coupling to a digital logic circuit. The circuit shown in FIG. 1 allows a narrow range of input voltages to safely drive a logic input of a digital circuit. A input voltage is applied to a series connected first current limiting resistor 102 and zener diode 104. The zener diode 104 is selected to limit a second voltage to a series connected second current limiting resistor 106 and an input light emitting diode (LED) of an optocoupler 108.
For example, if the zener conduction voltage of the zener diode 104 is selected to be 5.7 volts and a current of 5 milliamperes (ma.) is desired to flow through the LED portion of the isolation circuit 108, a resistance value for the second current limiting resistor 106 may be calculated as follows: R106=(5.7 volts−0.7 volts)/5 ma, resulting in a resistance value of 1000 ohms for the second current limiting resistor 106. The input voltage must be greater than 5.7 volts for the zener diode to provide the full 5.7 volts to the second current limiting resistor 106, less input voltage than that will reduce the current through the LED of the optocoupler 108. When the current through the LED of the isolation circuit 108 is reduced significantly, the optocoupler 108 becomes unreliable in transferring the presence of an input voltage to the logic circuit.
As the input voltage increases, the current through the first current limiting resistor 102 and zener diode 104 will correspondingly increase. This is not desirable since the wattage of both the zener diode 104 and the first current limiting resistor 102 must be sized for a worst case maximum input voltage. Also the current load presented to the source of the input voltage increases. For example, at an input voltage of 10.7 volts and a current through the first current limiting resistor 102 of 10 ma., the resistance necessary for the first current limiting resistor will be 500 ohms. If the input voltage is at 105.7 volts, current flowing through the first current limiting resistor 102 will be 200 ma. and the current through the zener 104 will be 195 ma. At this current value, the first current limiting resistor 102 and the zener 104 must be rated to have a power dissipation of at least 20 watts. Also the input voltage source must be capable of supplying a 20 watt load. This is highly undesirable and therefore limits the range of input voltages that can be safely handled without having to change the value of the first current limiting resistor 102.
Operating temperature variations will also affect the characteristics of the aforementioned components such that proper operation at a low end voltage will vary with temperature. In addition, higher input voltages and operating temperatures may cause one or more of the aforementioned components to malfunction or fail.